Methane hydrate has attracted attention as a next-generation resource, and many researchers have conducted various studies to estimate its productivity. Numerical simulation is the optimal method for estimating methane gas productivity. Meanwhile, using a reasonable input parameter is essential for obtaining accurate numerical modeling results. Permeability is a geotechnical property that exhibits the greatest impact on productivity. The permeability of hydrate-bearing sediment varies based on the sediment pore structure and hydrate saturation. In this study, an empirical permeability model was derived from experimental data using soil specimens from the Ulleung Basin, and the model was applied in numerical analysis to evaluate the sediment gas productivity and ground stability. The gas productivity and stability of hydrate-bearing sediments were compared by applying a widely used permeability model and the proposed model to a numerical model. Additionally, a parametric study was performed to examine the effects of initial hydrate saturation on the sediment gas productivity and stability. There were significant differences in the productivity and stability analysis results according to the proposed permeability model. Therefore, it was found that for accurate numerical analysis, a regional permeability model should be applied.

Gas hydrates have great potential as future energy resources. Several productivity and stability analyses have been conducted for the Ulleung Basin, and the depressurization method is being considered for production. Under depressurization, ground settlement occurs near the wellbore and axial stress develops. For a safe production test, it is essential to perform a stability analysis for the wellbore and hydrate-bearing sediments. In this study, the development of axial stress on the wellbore was investigated considering the coupling stiffness of the interface between the wellbore and sediment. A coupling stiffness model, which can consider both confining stress and slippage phenomena, was suggested and applied in a numerical simulation. Parametric analyses were conducted to investigate the effects of coupling stiffness and slippage on axial stress development. The results show that shear coupling stiffness has a significant effect on wellbore stability, while normal coupling stiffness has a minor effect. In addition, the maximum axial stress of the well bore has an upper limit depending on the magnitude of the confining stress, and the axial stress converges to this upper limit due to slipping at the interface. The results can be used as fundamental data for the design of wellbore under depressurization-based gas production.

Under the seabed, gas hydrates are buried as solid materials that consist of water and gas molecules. Dissociation of gas hydrates induce mechanical properties change because solid-phase gas hydrates transformed to gas and liquid phase. Due to dissociation of gas hydrates, the bearing sediments can be collapsed or subsided. The triaxial test is a method for evaluating the mechanical properties of soil. Confining stress is applied to the specimen for simulating in-situ condition of the soil and axial loading is applied to induce shear failure. The modulus value can be derived through the relationship between the applying load and the strain level. Also cohesion and friction angle can be obtained based on the peak strength value according to various conditions of the confining stress. In the case of gas hydrate-bearing sediments, the mechanical properties change by the cementation effect of the gas hydrates. Therefore, experimental research for mechanical properties of gas hydrate-bearing sediments are required to understand mechanical behaviors of the sediments. However, high pressure and low temperature conditions are necessary to maintain stable condition of gas hydrates during the experiments. The triaxial tests should be conducted under the gas hydrate stable environment. In this study, in order to simulate the gas hydrate-bearing sediments, we constructed a system that can perform triaxial tests under high pressure and low temperature environment. Then, triaxial tests were carried out using specimens of gas hydrate-bearing sediments. Mechanical properties that achieved from the triaxial tests can be used as input parameters for the numerical analysis, which simulate the gas hydrate dissociation process.